This invention is directed to a field emission liquid metal ion source which has a point which is resistively heated and which is coated with liquid metal to emit ions.
The first liquid metal ion source described in the literature was designed and developed by Clampitt at Culham Laboratories in England. It is shown in U.K. patent No. 1,442,998. Subsequently to that publication, Culham Laboratories marketed liquid metal ion sources of the same general configuration intended for producing ions of copper, silver, gold, bizmuth, lead, tin, indium, gallium, uranium, mercury, silicon, germanium, iron, aluminum, lithium, sodium, potassium, rubidium, and cesium. In the United States, Dublier Scientific marketed a similar liquid metal ion source, and it is believed that all Dublier Scientific sources were made of refractory metals. The early sources with low melting point fuels were radiatively heated via a simple external coil. Later, the sources were oven heated for use with higher melting point materials.
The gas field ionization "hairpin" ion source was originally designed for gas field emission. The heart of the hairpin device is a U-shaped heater wire with a needle welded to the apex of the U. The heater wire is used to clean the attached needle by heating it to cause outgassing. When used with liquid metal, the hairpin ion source works fairly well with non-corrosive fuel materials. L. W. Swanson first used the device as a liquid metal source by applying liquid metal directly to the needle. A number of drawbacks are found. The hairpin device is difficult to make, due to the necessary welding of the needle to the U-shaped heater wire. Since the needle is mounted on the heater wire and the heater wire is employed for structural support of the needle, the ion source lacks stability in the direction perpendicular to the plane of the U-shaped heater wire when the heater wire is heated. In addition, the hairpin source is thermally inefficient and has a poor temperature gradient.
Advances in liquid metal ion sources have been made at Hughes Research Laboratories division of Hughes Aircraft Company. On behalf of Hughes Research Laboratories, Jerg B. Jergenson invented the structures represented in U.S. Pat. Nos. 4,318,029 and 4,318,030. These sources are easy to make, inexpensive and reliable. As a result, considerable advances in the employment of ions from a liquid metal source have been achieved. Ions have been produced from fuel alloys. However, it soon became evident that alloys containing boron attacked the metallic source components used in the construction of the sources illustrated in those U.S. patents. When such liquid metal ion sources are made of non-metallic materials, they are difficult if not impossible to make. In addition, the needle may be inefficiently heated and the required temperature gradient may not be achieved when the source structure is made of non-metallic materials.
Boron is one of the most important doping elements for silicon devices. However, fabrication of a metal liquid metal ion source for utilization of liquid boron has been impractical due to the high melting point of metallic boron and the strong corrosive effect of the boron on most metals. To decrease the problems associated with the high melting point of metallic boron, a rhenium needle and eutectic alloys containing boron have been used. However, the lifetime of some sources employing boron containing eutectic alloys have been restricted to about 10-15 hours due to the corrosion of the rhenium emitters.
One group in Japan has attempted to find a boron containing alloy which is non-corrosive, but substantial lifetimes have not found. See "Liquid Metal Alloy Ion Sources for B,Sb, and Si," by K. Gamo, published in Journal of Vacuum Science Technology, Volume 19, No. 9, November/December 1981, pages 1182-1185. Further development work in Japan uses previously developed glassy carbon emitters for liquid metal ion sources and has used such sources with nickel boride as a fuel material. A lifetime of 200 hours for this type of source has recently been quoted. However, it has been admitted that the nickel constituent of the nickel boron alloy corrodes the emitter tip. Such sources have been used as an ion source in a mass-separating column for ion implantation. This is discussed in a publication "Mass-Separated Microbeam System with a Liquid-Metal-Ion Source," by T. Ishitani, et al. published in Nuclear Instruments and Methods in Physical Research, Volume 218 (1983) pages 363-367. The entire disclosure of this background material is incorporated herein by this reference.
The ion source is a very small and delicate structure. Furthermore, the emission point must be as positively located as possible in order to maintain adequate alignment of the emitted ion beam. Thus, there is a need for an improved field emission liquid metal ion source.